turbo boost vs super boost (not a turbo vs. super)

[njp98]

ok, here's my question. Is 10psi boost on a SC the same as 10psi boost on a turbo? Shane runs 7psi and ran 12.9. That seems like he has a better time than what the 10psi SC kit would give. Is it possible then that since 10psi is ok to run with a SC on a 3.8 , could you run 10psi (safely on stock internals) with a turbo on a 3.8? I know about the parasitic loss and such and the advantages of turbo vs. SC - is this the only thing adding that extra power or what?

[Radcat]

I have been running 10psi the last month or so with my turbo.

[njp98]

Whats your general feelings with the 10psi turbo? Any problems? Times? Dyno? I'd like to know because that sounds awesome. Also how are your turbo headers constructed?

[xjarayax]

Shane also has the supporting mods to get him to those times, torque converter, nittos, etc.

[HAZ-Matt]

The torque converter is going to make a world of difference for both the supercharger and the turbo. But in general, all things being equal, 10psi for both would translate into the same cubic feet per minute. However, depending on your compressor efficiency, the intake temperature can vary widely, giving a different flow rates of kg/minute. The compressor of a supercharger is only at it's peak efficiency at 1 particular engine speed since it is directly connected to the crank via a pulley and gearbox, whereas a turbo compressor is at it's peak efficiency much longer, as it's speed is regulated by the wastegate.

[Mikael]

Take a supercharger and a turbo that flow the same rate (lb/min) and have the same max psi, the turbo will always be faster. No, not because of parasitic loss, that's pretty negligable. A centrifigal blower will make its max psi at redline, while the turbo is very low, like 3k rpm, so its power band is much fatter.

As far as blowing your engine, you have two concerns. First is detonation, which isn't related to power. The second would be total cylinder pressure, which isn't related to boost (directly).

[mattsv6]

it all depends on size of the impeller's and housings. cubic feet per minute of air will always be different with a change in anything on any type of compressor. try too map the shaft speeds after the gear reduction with a blower and match that too when the exhaust wheel reaches full spool and that is the best way too compare these two.

[98GREENGLOW]

That's what I was thinking as well... Now I know jack about turbos but I do know that if a centerfigal blower is making 9 psi like my powerdyne for example that I wont see that 9 psi until 5k rpms. Now in first gear it goes by so guick that you literaly get that boost the second your foot slams the pedal. Then in between gears it goes back down to zero and then builds back up to 9 psi in second gear . That one takes a second or two ,too reach the full psi just like it takes your car a little while to get to 5 k in second gear. In third gear Ive never even seen anything more than 71/2 psi cas it builds so slow (just like the gear)that I would probaly have to be doing like 140mph to even see it. Now keep in mind that you dont actualy see the psi start from zero...It gets to 5 psi before you can even blink and then builds to 9 psi at a speed depending on the gears I just described above. -----------------------------------------------------------------------------------------------------------------------------------I thought the turbos would do exactly the same thing though...The only differance would be that instead of loosing power in between the gear changes due to the pully, it would loose power in between gear changes due to the exhaust.................OR , those it build up to a certain rpm and then deliver the 9 psi through out the entire rpm range through all gears?????? If thats the case then having a 7 psi turbo would on average would be equal to a charger making 10 psi when you averaged out what amount of boost that was actualy averaged through the hole 1/4 mile on each system. -----------------------------------------------------------------------------------------------------------------------------------Can someone explain exactly how the turbo maintains it's boost??

[njspder]

<blockquote>quote:</font><hr>Originally posted by Mikael:
[QB]Take a supercharger and a turbo that flow the same rate (lb/min) and have the same max psi, the turbo will always be faster. No, not because of parasitic loss, that's pretty negligable. QB]<hr></blockquote>

a good centrifugal type blower still wastes about 9-11% of the power it produces. teh higher the boost level the greater teh loss of efficiancy.

i have trouble considering that negligable.

the parasitic waste is exactly what make sthe difference when comparing equal boost levels.

later
tim

[xjarayax]

Well let's think of it like this..... you can spin the pulley of a supercharger with your hand, and your hand generates how much horsepower? I don't think it robs that much off the crank as people make it out to be.

[Ike]

<blockquote>quote:</font><hr>Originally posted by 98GREENGLOW:
Can someone explain exactly how the turbo maintains it's boost??<hr></blockquote>

This explanation details the boost TRENDS so the actual numbers may be different than personal experience. Please read accordingly.

The difference is like this:

A supercharger is directly connected to the engine via the crank pulley. When designing the supercharger system, the manufacturer must choose components so that boost is limited to what the motor can handle at maximum rpm (a little more than redline). The two common superchargers (roots and centrifugal) make boost in two different ways. The roots type (and those like them-positive displacement) make more or less the same boost for the entire rpm range (forgiving some time when waiting for the entrainment of the air ~2000rpm) so the 6psi it makes at redline is only a little more than the ~5psi it makes at 3000rpm. A centrifugal produces boost more gradually, so that there is significant difference between onset (~2psi@2500, or something like that) and redline (~6psi@6500, say) for the boost curve. So when we say 6psi when talking about a centrifugal, we mean 6psi at redline, but when we are talking about a roots, we can be very near 6psi sooner than redline.

OK, now turbochargers are a little different. Instead of being linked directly to the engine via the crank, they are powered by the expansion and flow of exhaust gases. Instead of a pulley, there is a turbine that is connected to the compressor. This turbine requires a certain amount of flow before the turbo will create meaningful boost (i.e. turbo lag). Two questions can be answered here. First, this is why turbochargers are more efficient than even centrifugal superchargers. It is "cheaper" (in terms of power) to spin this turbine with hot expanding exhaust gas (which would just be wasted any way) than to spin the pulley on the supercharger. Yes, yes, it is easy to spin by hand, but take the force you apply with your hand, multiply by 6500rpm redline and then tack on some extra power to actually compress something--then you'll see why some power is actually needed--it's a big deal. Secondly, the exhaust flow through the turbine housing is controlled by a wastegate. By allowing all of the exhaust gas to flow through the turbine section, the turbo will be able to reach maximum boost sooner in the rpm range than a centrifugal supercharger. Once boost nears an allowable limit, the wastegate opens and directs some of the exhaust out of the tail pipe instead of through the turbine. This is how the turbocharger maintains higher boost levels than a centrifugal turbocharger once it has started making boost. It is tuning dependent (as well as how good your wastegate is--imagine if it got stuck) to say when boost starts, but you can have that same boost all the way to redline if you do it right.

Sorry for the long dissertation, but you asked. If you want the cliff notes (or some proofreading), I will provide later after some sleep.

I hope this clears up some of the confusion.

[Tiago]

<blockquote>quote:</font><hr>Originally posted by Ike:


This explanation details the boost TRENDS so the actual numbers may be different than personal experience. Please read accordingly.

The difference is like this:

A supercharger is directly connected to the engine via the crank pulley. When designing the supercharger system, the manufacturer must choose components so that boost is limited to what the motor can handle at maximum rpm (a little more than redline). The two common superchargers (roots and centrifugal) make boost in two different ways. The roots type (and those like them-positive displacement) make more or less the same boost for the entire rpm range (forgiving some time when waiting for the entrainment of the air ~2000rpm) so the 6psi it makes at redline is only a little more than the ~5psi it makes at 3000rpm. A centrifugal produces boost more gradually, so that there is significant difference between onset (~2psi@2500, or something like that) and redline (~6psi@6500, say) for the boost curve. So when we say 6psi when talking about a centrifugal, we mean 6psi at redline, but when we are talking about a roots, we can be very near 6psi sooner than redline.

OK, now turbochargers are a little different. Instead of being linked directly to the engine via the crank, they are powered by the expansion and flow of exhaust gases. Instead of a pulley, there is a turbine that is connected to the compressor. This turbine requires a certain amount of flow before the turbo will create meaningful boost (i.e. turbo lag). Two questions can be answered here. First, this is why turbochargers are more efficient than even centrifugal superchargers. It is "cheaper" (in terms of power) to spin this turbine with hot expanding exhaust gas (which would just be wasted any way) than to spin the pulley on the supercharger. Yes, yes, it is easy to spin by hand, but take the force you apply with your hand, multiply by 6500rpm redline and then tack on some extra power to actually compress something--then you'll see why some power is actually needed--it's a big deal. Secondly, the exhaust flow through the turbine housing is controlled by a wastegate. By allowing all of the exhaust gas to flow through the turbine section, the turbo will be able to reach maximum boost sooner in the rpm range than a centrifugal supercharger. Once boost nears an allowable limit, the wastegate opens and directs some of the exhaust out of the tail pipe instead of through the turbine. This is how the turbocharger maintains higher boost levels than a centrifugal turbocharger once it has started making boost. It is tuning dependent (as well as how good your wastegate is--imagine if it got stuck) to say when boost starts, but you can have that same boost all the way to redline if you do it right.

Sorry for the long dissertation, but you asked. If you want the cliff notes (or some proofreading), I will provide later after some sleep.

I hope this clears up some of the confusion.<hr></blockquote>


turbo chargers spool and build boost according to engine LOAD, not only RPM. Its more of a volumetric flowrate of exaust gas that dictates spool. You can be at 5-6000 rpm and have ZERO boost if you don't have any load on the engine IE in neutral.

now 2-3000 rpm from a roll and hit the gas, full boost will come with the quickness on a properly built turbo setup. This is why 2 step rev limiters ownzjoo. I can sit at the staging light and bounce off the limiter at WOT and build boost with the clutch engaged, and wait for the green with about 3-4 lbs of boost, the SECOND I drop the clutch and any load is transfered through the drivetrain into the motor, I will already be at full boost (10-15 psi depending), its a beutiful thing. It will never drop boost throughout the run either, even when u shift ;)

[ November 10, 2003: Message edited by: Tiago ]</p>

[Ike]

<blockquote>quote:</font><hr> turbo chargers spool and build boost according to engine LOAD, not only RPM <hr></blockquote>

Yeah, I agree, but the problem now is explaining how the turbocharger "knows" the difference between load and rpm. Since the turbine section relies on exhaust gas flow--well, I guess most people would ask:

Wouldn't exhaust flow increase with rpm? And how is that different than load?

I don't know how to go about explaining the difference (without saying "it just does"), so you try! ;)

[ November 14, 2003: Message edited by: Ike ]</p>

[slow]

lots of good explanitions above, but here is basic.

Supercharger, you get peak numbers at redline. (or close to it)

Turbo, a well sized turbo setup will be full boost (and maintain that to redline) at about 3000 rpms or so.

Turbo usually equals more area under the curve, which is what really determines power.

Ryan.

[Tiago]

<blockquote>quote:</font><hr>Originally posted by hou-tex-six:



because at part thorottle per say you are only getting X amount of air CFM going into the engine, and therefore also out of the engine. At WOT now you have 2X lets say. So you have twicee as much air going out , more air = more spool

therefore you can be at low rpm and still be at WOT with alot of air going through the motor, causing the turbo to spool.<hr></blockquote>


me again
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